The present invention relates to nuclear reactor plants and, more particularly, to pressure containment cooling systems for such plants. A major objective of the present invention is to provide for simplified pressure containment cooling and heat removal for a boiling water reactor plant both in isolation condensor (IC) mode and in a passive containment cooling system (PCCS) mode, which can be triggered by a loss of coolant accident (LOCA).
Full utilization of nuclear energy awaits the development of nuclear reactor plants that resolve safety issues at reasonable cost. A typical nuclear reactor generated heat through the fissioning of a core containing nuclear fuel. This heat is transferred from the core by a collant that circulates in a reactor vessel containing the core. The heated fluid is directed to a transducer to convert the energy into an alternative form. For example, the heated fluid can be used to drive a turbine, which in turn drives a generator to produce electricity.
Safety standards for nuclear reactors require an ability to handle predefined hypothetical accidents. A nuclear reactor plant should be able to respond to a LOCA, such as would result from a break in a conduit carrying coolant to or from the reactor. As a further precaution, the plant should be able to prevent radioactive release to the environment even when an active safety system fails during a LOCA. Thus, the PCCS mode should be able to function without human intervention or assistance for at least 72 hours after a LOCA.
Loss of coolant causes the temperature in the reactor to rise, boiling remaining coolant and causing the pressure in the reactor vessel to rise. The excess steam so generated escapes into the dry well in which the vessel is situated. The dry volume is much greater than that of the vessel, so this escape results in a considerable pressure relief. Under normal conditions, the dry well is filled with a nonreactive gas such a nitrogen of air. After a LOCA, the stream escaping from the reactor vessel increases the dry well pressure. This pressure accumulation is undesirable since excessive pressure increases the probability that radioactive gases might be forced into the environment.
The increasing dry well pressure forces open a passage to a wet well, which is a large chamber containing a "supression" pool of water. Some of the dry well gas, including the steam, escapes into the wet well. This reduces the dry well pressure. The water in the wet well condenses stream, keeping the wet well pressure low.
Within minutes after a LOCA, a gravity-driven coolant system (GDCS) dumps another pool of water into the reactor to help cool the core. This added water is then converted to stream which increases pressure in the vessel and dry well again. This pressure can be relieved by escape into the wet well. However, as the internal pressure of the wet well increases, its ability to relieve pressure in the dry well decreases. The capacities of the GDCS and the wet well are designed to handle the demands of the plant for the critcal first fifteen minutes or half hour of PCCS mode operation. After this time, the GDCS is exhausted and the port between the wells remains closed to further gas evacuation. The port is closed since the pressure differential between the wells no longer displaces the water covering the port.
The GDCS coolant slows the rate of stream generation and decreases the pressure in the vessel and dry well. When the pressure in the dry well falls below that of the wet well by a threshold differential amount, a vacuu, breaker releases gases from the wet well to the dry well. This clamps the pressure differential of the wet well to the predetermined level above that of the dry well.
While these measures address short-term pressure accumulation, there is a need to handle longer term effects of decay heat. While controls rods are inserted soon after the onset of a LOCA to reduce core reactivity, core heat generation decreases only gradually. The heat produced during this decrease is termed "decay heat". For a period beginning a few minutes after the LOCA and extending over a period of about three days before human-assisted maintenance activities will be begun this decay heat must be dissipated. Otherwise, over time, the deacy heat might result in excessive pressure accumulation through boiling of coolant from the GDCS.
To mitigate this long-term pressure accumulation, a heat exchanger can be used to remove heat, and thereby pressure, from the vessel and dry well. The heat exchanger can transfer heat from the fluid from the reactor to a "condenser" pool of water. Unlike the suppression pool in the wet well, the condenser pool can be vented to the environment so that its temperature does not substantially exceed 100.degree. C. Thus, the cooling effectiveness of this heat exchanger can be maintained over a period of days. To provide redundancy, several such heat exchangers can be required.
One of the paradoxes facing the nuclear industry is that each system introduced to enchance the safety of a plant adds another component that can fail. In addition, each system adds to the volume and complexity of a plant. Since plant costs scale dramatically with plant volume, plant costs can be severely and adversely affected by the extra volume consumed by safety systems. In addition, complexity can adversely affect both cost and reliability. What is needed is a heat exchanger for dissipating decay heat in PCCS mode that adds minimal volume, complexity and cost to a reactor plant.